Condensation Heat Transfer Of Vapor Research Articles

Effect of non-condensable gas on the direct-contact condensation heat transfer of vapor in a convergent tube

The direct-contact condensation heat transfer of vapor in the presence of non-condensable gas (NCG) on the falling water column in a convergent tube was simulated with a two-phase boundary layer approach, and the variations of the condensation heat transfer parameters and the vapor condensation length were analyzed. Results show that the convergent angle has an important influence on the condensation heat transfer parameters. Both Nusselt and Stanton numbers rose as the convergent angle went up from 0° to 40°. The increment in the convergent angle enhanced the condensation heat transfer coefficient from 1.06 to 4.8 times at the range of the vapor pressure from 1 atm to 3.6 atm. The increase in the inlet NCG mass fraction caused a reduction in the condensation heat transfer parameters. and its accumulation in the downstream where its mass fraction was over 90 %. The nozzle diameter and the inlet velocity and temperature of falling water column affected importantly the condensation heat transfer parameters. The vapor condensation length was increased with the rising inlet velocity. of falling water column, the vapor pressure and the nozzle diameter, while decreased with the ascending of the inlet NCG mass fraction and the convergent angle. The increment in the vapor pressure enhanced effectively the condensation heat transfer parameters, and weakened the adverse effect of NCG on the direct-contact condensation heat transfer.

Graphene coating reduces the heat transfer performance of water vapor condensation on copper surfaces: A molecular simulation study

Water vapor condensation is a key process for many applications. Existing studies in water vapor heterogeneous condensation have used different methods to modify the hydrophobicity of the water vapor condensation surface. One way to modify the surface properties is to use graphene coating. On a macroscale, surface modification using graphene coating can improve water vapor condensation heat transfer of various substrates. However, on the molecular scale, its effect is poorly understood. This work investigated the effect of graphene coating on water vapor condensation using molecular dynamics simulations (MDS). We examined the water vapor condensation on bare copper surfaces considering the effects of initial temperature difference, water model, and surface size, parameters that have not been investigated by previous studies employing MDS for the same water-surface configuration. One water model was then used to simulate the condensation on a copper surface with and without graphene coating. We then investigated the effect of graphene defect, the energy and vibration of graphene atoms, and the interaction between graphene and copper layers. The surface size notably influenced the condensation rate and heat transfer performance. The condensation rate and heat transfer performance were significantly reduced when the copper surface was coated by graphene. The number of water molecules condensed was 1253 molecules/ns on the bare copper surface, compared to 587 molecules/ns on the graphene-coated copper surface. Moreover, the water molecules condensed on the graphene-coated copper surface tended to return to the bulk vapor phase. Other important results are also provided. This study gives an insight into the water vapor condensation on graphene-coated copper surfaces, useful to pursuit the design and optimization of graphene-coated copper surfaces for applications that need efficient water vapor condensation, such as for industrial applications, like thermal, chemical, and nuclear.

Vapor condensation heat transfer on two-tier hierarchical microstructured surface

Condensation heat transfer on the hierarchical microstructured surfaces is simulated using a three-dimensional phase-change lattice Boltzmann method. Parametric effects raised by widely varying primary micropillar spacing, secondary micropillar position and height, surface wettability and subcooling on the condensation heat transfer are discussed in detail. The condensation heat flux first increases but then decreases with the increase of spacing between primary micropillars. Droplets are prone to nucleate on the top sides of primary micropillars with smaller pillar spacing, and they tend to maintain the Cassie‐Baxter state when secondary micropillars are arranged on the lateral sides of primary micropillars. As the height of secondary micropillar increases, the heat flux first increases but then decreases thereafter. When the contact angle rises from 57° to 130°, the state of condensate droplet changes from the Wenzel state to the partial wetting state. It is also found that there exist four types of droplet nucleation sites on the hierarchical microstructured surface: at the bottom of secondary micropillars, at the corner between primary micropillars and substrate, between two adjacent primary micropillars, and at the center of substrate between four primary micropillars. The increase of subcooling is favorable for nucleation, and the heat flux and condensation rate increase in accordance.

Enhancement of vapor condensation heat transfer on the micro- and nano-structured superhydrophobic surfaces

Recently, micro- or nano- structured surfaces haven been developed to enhance condensation heat transfer, water harvesting and self-cleaning. However, at large subcoolings, condensate floods the subcooled substrate, thus deteriorating the heat transfer efficiency. Here, the superhydrophobic surfaces with micropillared and nanopillared structures are proposed to enhance heat transfer at large subcoolings. The influence of micropillar spacing and surface subcooling on the droplet dynamics and heat transfer performance is experimentally investigated using microscopic visualization techniques. In addition, the microscopic modeling of condensation heat transfer on the microstructured surfaces is performed using the mesoscopic lattice Boltzmann method. The results demonstrate that the droplet size distribution on the micropillared surface is significantly smaller over that of the nanostructured surface. The heat transfer coefficient decreases with the increase of micropillar spacing. As the subcooling rises, although the condensate floods the substrate, the heat transfer coefficient of the S10R30 (S10R30 represents the micropillar arrays with s = 10 μm and 2r = 60 μm) surface is enhanced by 26.4% compared to the hydrophobic nanostructured surface. This is because the height of liquid film is the same of order of magnitude as the micropillars, reducing the thermal resistance caused by the liquid layer. Combining environmental scanning electron microscope (ESEM) observations and LB simulation results, it is concluded that the droplets first nucleate at the bottom corner of micropillars. In addition, the condensate droplets merge to form a film, fill the micropillar gaps, and cover the entire micropillars, leading to a sharp decrease in heat flux. These findings provide a theoretical and experimental guidance for the development of condensing surfaces to enhance heat transfer.

Hierarchical microcavity topography for enhancement of water vapor condensation heat transfer by regulating droplet dynamics and droplet size distribution

Manipulating droplet dynamics is of great importance in condensation heat transfer enhancement. Cavity topography was believed to be able to regulate droplet merging and self-removal via Laplace force imbalance by the cavity wall and the effect of spatial confinement and isolation. However, the size effect of the microcavity on the droplet dynamics behavior, droplet size distribution and condensation heat transfer performance are not sufficiently understood. In this paper, hierarchical microcavity-arrayed superhydrophobic surfaces with a nominal cavity diameter of 20, 35, 50 and 70 μm were fabricated. Condensation experiments were first carried out in the atmosphere under an optical microscope, and then in pure steam in a self-designed environmental chamber at 4000 Pa. Local dynamic behaviors and time-lapsed evolutions of droplets during condensation were observed. Droplet size distribution and jumping rate were statistically analyzed using ImageJ software. It was proved the smaller cavity size of 20 μm is beneficial to facilitate condensate droplet jumping and increase the density of small droplets. The surface coverage ratio on the microcavity-arrayed superhydrophobic surfaces was only half of that on the hydrophobic surface at steady state. Enhancement of pure steam condensation heat transfer can be realized by effectively regulating droplet dynamics using microcavity topographies at limited subcooling degrees.

Lattice Boltzmann Modeling of Condensation Heat Transfer on Downward-Facing Surfaces with Different Wettabilities.

The model of vapor condensation heat transfer on downward-facing surfaces with different wettabilities is built by a two-dimensional (2D) lattice Boltzmann method. Dynamic evolution of condensate microdroplets on different wettability surfaces is simulated and the influence on heat transfer performance is analyzed. Moreover, the mechanism of a heterogeneous wettability surface enhancing condensation heat transfer is explored by investigating the condensate behaviors in the process of condensation. The numerical results indicate that as the contact angle of the homogeneous wettability surface increases, the initial nucleation time of the condensate is prolonged, while the departure time of the condensate is reduced significantly. The temperature adjacent to the gas-liquid interface, especially in the three-phase contact line region, is much higher than elsewhere due to the release of latent heat during condensation. Coalescence and detachment behaviors of condensate droplets cause the average heat flux to fluctuate locally with time. For the hybrid wettability surface, if the proportion of hydrophobic regions is small, the condensation heat transfer performance will be deteriorated. However, increasing the hydrophobic-hydrophilic ratio has a positive effect on enhancing heat transfer. It is found that a critical hydrophobic-hydrophilic ratio exists to optimize the heat transfer performance. For the gradient wettability surface, directional migration induced by capillary force facilitates the removal of condensate droplets, thereby enhancing the condensation heat transfer. Furthermore, a larger wetting gradient benefits to further improve the heat transfer performance. The results are valuable for optimally designing the heat transfer enhancement of vapor condensation on functionalized surfaces with heterogeneous wettability.

Effect of Polymer Coating on Vapor Condensation Heat Transfer

Abstract Condensation heat transfer coefficients (HTCs) are rather low compared to thin film evaporation. Therefore, it can be a limiting factor for designing heat transfer equipment. In this work, heat transfer characteristics of water vapor condensation phenomena were experimentally studied on a vertically aligned smooth copper substrate for a range of pressures and temperatures for two different liquid wettability conditions. The heat transfer performance is dominated by the phase change process at the solid–vapor interface along with the liquid formation mechanism. Compared to heat transfer results measured at an untreated copper surface, heat transport is augmented with a thin layer of perfluoro-silane coating over the same substrate. In this work, the effect of saturation pressure on the condensation process at both surfaces has been investigated by analyzing heat transfer coefficients. The results obtained experimentally show an increase in contact angle (CA) with the surface coating. A heat transfer augmentation of about 26% over uncoated surfaces was obtained and surfaces did not show any degradation after 40 h of operation. Finally, current results are compared with heat transfer values reported in open literature.

Effects of stagnant air on condensation heat transfer over a horizontal tube

The effects of stagnant air on the condensation heat transfer of vapor over a horizontal tube were investigated by experiments. The condensation wall temperatures along the circumference and the cooling water temperature were measured at various cooling water and vapor mass fluxes as well as vapor pressures. The heat transfer coefficients and Nusselt number were calculated, and the effects of stagnant air on several parameters, including the wall temperature of the condensation tube, Nusselt number, heat transfer coefficient, and flow pattern transition point of liquid film, were analyzed. Results showed that stagnant air was a main factor that affected the heat and mass transfer uniformity of vapor. Unlike in pure vapor condensation, the temperature difference between the top and bottom parts of the condensation tube increased when the mole fraction of stagnant air was less than 0.17, whereas it decreased when the mole fraction of stagnant air was larger than 0.47. The increment in the mole fraction of stagnant air reduced the temperature difference along the circumference of the condensation tube wall. As the mole fraction of stagnant air increased, the heat transfer coefficient and Nusselt number decreased. However, Nusselt number was insensitive to the effect of air when the mole fraction of stagnant air was larger than 0.25. As the circumferential angle of horizontal condensation tube increased from 0° to 180°, the Reynolds number of liquid film transformed from the laminar state to the turbulence state and then changed into the laminar state. Reynolds number reached a maximum value within the circumferential angles of 45°–90°. As the circumferential angle changed from 90° to 180°, a large amount of condensate fell, and the liquid film attached to the condensation tube wall became thinner, which resulted in an intensive reduction in the Reynolds number of the liquid film. As the mole fraction of stagnant air increased, a transition point of liquid film moved to a larger circumferential angle, which delayed the transition of liquid film from laminar flow to turbulence. The present experimental results were compared with the data from Sherkriladze and Gomelauri (1966), Rose (1984), Fujii et al. (1972), and Nusselt (1916).

Development of an analytical model for pure vapor downflow condensation in a vertical tube

In-tube condensation of pure vapor are widely adopted by passive safety systems of advanced nuclear reactors to remove the decay heat. Accurate calculation of the in-tube condensation heat transfer coefficient is important for evaluating the heat removal ability. A better understanding of the mechanism of pure vapor condensation heat transfer and its modelling are essential for the design and assessment of those passive safety systems. An analytical model for pure vapor condensation in a vertical tube is developed based on mass, momentum and energy conservation equations. The total axial pressure drop is calculated through the momentum equation of the vapor core assuming a uniform radial pressure distribution. An optimized transition area between the laminar and turbulent film flow is defined to avoid discontinuity of heat transfer coefficients. Based on the sensitivity analyses, Kay’s turbulent eddy diffusivity model and his turbulent Prandtl number model were recommended for the newly proposed model. The predicted results were compared with results of Shah (2016a,b), Chen et al. (1987) and Cavallini et al. (2002) correlations as well as Kuhn’s and KAIST experimental data. It was found that the proposed model predicts the heat transfer coefficient well with a mean absolute deviation of 11.99% for Kuhn’s data and 8.08% for KAIST’s data.

Visualization study of the influence of non-condensable gas on steam condensation heat transfer

This paper experimentally investigates the influence of non-condensable gas on water vapor condensation heat transfer on an isothermal vertical aluminum plate in an air mass fraction range of 0.05–0.5. A high-speed digital camera was employed to capture real-time images of water vapor condensation in non-condensable gas under different wall temperatures and air concentrations. The experimental results showed that the condensation modes varied with surface subcooling and air concentration. Four condensation modes were observed in the experiments: drop, drop-streak, film, and streamlet. Under identical air concentrations, the condensation mode varied from the fine drop and film mode to the larger drop-streak mode; the condensation drop diameters departing the plate became obviously bigger, the drops fell off the plate more quickly, and the heat transfer coefficients decreased gradually as surface cooling increased. Under identical surface cooling conditions, the condensation drops grew and fell off more quickly when the air mass fraction was low, the condensation drops irregularly varied from drop, drop-streak, to ribbon stream, and the condensation had a tendency to spread out. The heat transfer coefficients decreased with the increase of the air mass fraction. The heat transfer coefficient declined greatly under a lower air mass fraction (5–10%), while it declined slightly under a higher air mass fraction (40–50%).

Analysis of Annular and Stratified Flows for Vapor Condensation Heat Transfer in Horizontal Tubes

The greater part of the horizontal condenser tube is occupied by the stratified and annular flows, which play important roles in condensation heat transfer coefficients. Both a volume of fluid (VOF) interface tracking method and k-ε two equations model was applied to analyze the characteristics of the stratified and annular flows for horizontal tubes, obtaining distribution of velocity, contours of both temperature and local condensation heat transfer coefficients. The computation shows that an annular flow having thinner liquid film attached on the inner surface of tubes appears at the inlet of the horizontal condenser tube because vapor exerts a higher interfacial shear stress on the gas-liquid interface, therefore, there are a higher local condensation heat transfer coefficients there. Next, as vapor quality decreases and condensate gravity increases along the condenser tube length, the condensation flow pattern transforms from the annular flow into the stratified flow in which local condensation heat transfer coefficients decreases and distributes unevenly along the circumference as liquid film at the bottom of the condenser tube become thicker. In addition, in the stratified flow, a wave structure is formed in the middle of the condensate pool at the bottom of the condenser tube because condensate on the top of the condenser tube slides into the bottom of the condenser tube and collapse each other. The heat transfer rate calculated by the present method is compared to those predicted from a Shah correlation, the agreement is found to be good, and both errors is within 7%.

Analysis of Stagnant Non Condensable Gases Effects on Condensation Heat Transfer in a Vertical Tube

In a vertical condenser tube installed at the cold end of a non-vacuum separate type heat pipe, non condensable (NC) gases in the system is pushed by continuous vapor flowing from the hot end into the condenser tube at the cold end, gathering above condensate at the outlet of the condenser tube. Therefore, condensation heat transfer of vapor with the stagnant NC gases occurs in the condenser tube. It is necessary to comprehend the effects of stagnant NC gases on condensation heat transfer. A VOF method was adopted to analyze how stagnant NC gases affect condensation heat transfer, a mass fraction equation of NC gases was used to solve diffusion between NC gases and vapor, a Hertz-Knudsen-Schrage model was applied to deal with condensation rate of vapor on the surface of liquid film. Parameters, including volume fraction, velocity, pressure, mass fraction of NC gases and condensation heat transfer coefficients (HTC), were obtained. Results show that a lot of NC gases deposits in the condenser tube rear, leading a lot of vapor to condense at the condenser tube front. NC gases slightly affect condensation HTC of the tube front, and severely degrade condensation HTC of the tube rear. Furthermore, an increase in mass of NC gases causes a rise in pressure and velocity, improving condensation heat transfer.

Mathematical simulation of steam condensation with high fractional noncondensable gas in the dewvaporation desalination process

A steady-state mathematical model was developed for vapor condensation heat transfer process with high fractional noncondensable gas by taking into account the heat and mass balances on both sides of the condensing column. Meanwhile, the condensation mass transfer coefficient at the condensing side was correlated along the height of the condensing column based on the large amount of experimental data. The correlations represented the experimental data very well, which showed that the model is reasonable. Mathematical simulation results present various column height profiles including temperature of mixed gas and condensing heat transfer coefficient, noncondensable mass fraction and condensate mass flow rate

Vapor Condensation Heat Transfer in a Thermoplate Heat Exchanger

AbstractThe heat transfer and pressure drop in a thermoplate heat exchanger operating as a condenser have been investigated experimentally. In order to separate the heat transfer resistances in the condensation process, the single phase forced convection has been studied using distilled water and Marlotherm oil in the thermoplate and correlations developed for the Nusselt number and the friction factor. For the condensation experiments, an apparatus has been constructed comprising two identical condensers composed of the same thermoplate type as employed in the single phase experiments. Isopropanol is used as a test fluid at pressures below atmospheric pressure. The heat transfer resistances in the condensation experiments are separated and expressions for the condensation heat transfer and pressure drop are developed with the aid of the results obtained in the single phase studies.

A Generalized Diffusion Layer Model for Condensation of Vapor With Noncondensable Gases

The diffusion layer model for condensation heat transfer of vapor with noncondensable gases was originally derived on a molar basis and developed from an approximate formulation of mass diffusion, by neglecting the effect of variable vapor–gas mixture molecular weights across the diffusion layer on mass diffusion. This is valid for gases having a molecular weight close to that of the vapor or for low vapor mass transfer rates, but it may cause serious error if a large gradient in the gas concentration exists across the diffusion layer. The analysis herein shows that, from the kinetic theory of gases, Fick’s law of diffusion is more appropriately expressed on a mass basis than on a molar basis. Then a generalized diffusion layer model is derived on a mass basis with an exact formulation of mass diffusion. The generalized model considers the effect of variable mixture molecular weights across the diffusion layer on mass diffusion and fog formation effects on sensible heat. The new model outperforms the one developed by Peterson when comparing with a wide-ranging experimental database. Under certain limiting conditions, the generalized model reduces to the one developed by Peterson.

Study on Direct Condensation Heat Transfer in Steam Injector

The characteristics and the performance limit of a steam injector have been investigated. The disruption of a 5 mm diameter water jet that flowed coaxially into a pipe of 7, 10 or 20 mm diameter where vapor flow existed and the heat transfer of vapor condensation on the jet surface were examined. The heat transport to the radial direction in the jet was stronger than that of the turbulent flow in a pipe. The condensation heat transfer was mainly dominated by the heat transport in the water jet since the vapor condensation heat transfer efficiency was quite high. If the Kelvin-Helmholtz instability wave length was used as the jet disruption criterion, it predicted the jet disruption conservatively. It was proved that the steam injector tested could elevate the delivery pressure to five times higher than the suction-side water pressure in the present experimental condition.

An experimental study on latent heat recovery of exhaust wet flue gas

The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture.